1. Field of the Invention
The invention relates generally to radiometric devices, and more particularly to devices which provide for radiant flux measurement of visible spectrum radiation through utilization of a response characteristic that allows an absolute measurement.
2. Description of the Prior Art
Radiometric devices producing measurements responsive to the radiant flux impinging thereon have long been available for measurement of light beams such as laser beams. These devices fall into one of two categories. One category includes absolute measuring devices, for which the relationship between the radiant flux of the light beam and the indicated measurement is based on fixed physical constants, and thus constitute primary measuring devices. The other category generates responses merely related to the radiant flux of the light beam by empirical data, and must be calibrated. Such devices are calibrated against primary measuring devices (absolute measuring devices) and are referred to as secondary measuring devices.
Absolute measuring devices for determining radiant flux have tended to be complicated and expensive, and involve physical constants such as the freezing point of platinum. Until a few years ago, the idea of using photodiodes as the sensing mechanism for an absolute radiometric device appeared unrealistic. An absolute response in photodiodes is possible, in that one photon will release one electron-hole pair in a silicon photodiode. But conventional photodiodes exhibited properties influenced by recombination of electrons and holes within its structure. The consequence of such recombination is that the actual charge flow through the photodiode is not related simply to the photons absorbed by the photodiode, but is also influenced by the recombination rate. The effect of such recombination is that the actual current flow external to the photodiode in response to the radiant flux absorbed by the photodiode is less than the theoretical current which would flow if one electron-hole pair were released for each photon absorbed by the photodiode. The ratio of the actual current flow to the theoretical current flow, if one electron-hole pair were released for every photon absorbed by the photodiode, is referred to as the internal quantum efficiency of the photodiode.
An additional problem with attempting to utilize silicon photodiodes in an absolute radiometric device is the reflection of light from the surface of the photodiodes. Although special processes involving nonreflective oxides reduce the reflective nature of the silicon, a reflective coefficient of 0.25 is still typical. The effect of such reflection of light is that the number of electrons released as a result of the absorbtion of photons is smaller than the number of photons which impinge upon the surface of the photodiode. The ratio of number of photons absorbed by the photodiode to the total number of photons impinging on the surface of the photodiode is referred to as the external quantum efficiency.
The consequence of these two sources of inefficiency is that a correction factor must be used to compensate for both when a conventional photodiode is used in a radiometric device. The correction factor is empirically determined, and the device is appropriately calibrated to the correction factor against some absolute measuring device standard.
The development of inversion layer photodiodes resulted in the availability of photodiodes which exhibit practically no recombination, i.e., have an internal quantum efficiency of essentially 100 percent. These inversion layer photodiodes were discussed in an article by T. Hansen in Physica Scripta, Volume 18, page 471, 1978. The result of reaching essentially 100 percent internal quantum efficiency is that it put the idea of creating an absolute radiometric device utilizing photodiodes within the realm of the feasible. It is necessary, however, to overcome the problems associated with the external quantum efficiency.